1. Field of the Invention
The present invention relates to a power source apparatus such as a so-called switching regulator for switching a DC input to obtain a desired DC output and to stabilize the DC output and a DC-to-DC converter for transforming a DC voltage and, more particularly, to a power source apparatus suitable for a digital processing apparatus having an internal clock signal with a predetermined frequency.
2. Description of the Prior Art
A switching power source such as a switching regulator is often mounted on a digital processing apparatus such as a digital audio equipment as a DC power source in consideration of size, heat generation, and the like. In this case, a switching frequency of the switching power source is set to be independent from a master clock frequency, a signal processing sampling frequency, and the like in the digital equipment on which the switching power source is mounted.
For this reason, in a conventional digital processing apparatus, a switching frequency of the switching power source is often different from a master clock frequency of the digital processing apparatus or a frequency obtained by dividing the master clock frequency. Leaked signals having different frequencies cause a beat which generates disturbing noise in an audio band.
In particular, a sampling signal having a frequency relatively approximate to an audio frequency of, e.g., 48 kHz or 44.1 kHz is used in the digital audio equipment. Therefore, it is highly possible that a frequency of a beat which is a difference between the sampling signal or its harmonic and the above-mentioned switching frequency or its harmonic may be an audio frequency.
In the digital audio equipment, a sampling frequency is often varied for the purpose of pitch control or the like. In this case, however, since a beat frequency determined in accordance with the sampling frequency and a switching frequency of the switching power source changes, disturbing noise is often generated.